Pressure-carrying pipes and other components are used in many technical areas such as for example in high-pressure technology and in the chemical industry. A distinction is drawn between a static pressure loading (remaining the same) and a dynamic pressure loading (alternating or pulsating). In many uses pressure-carrying components are frequently exposed to an alternating or pulsating pressure loading which, depending on the loading level, leads to more or less rapid failure of the component. Influencing values in respect of the loading are for example the pressure collective, the frequency of the pressure reversal alternations and the pressure delta (difference between maximum and minimum pressures). Dynamic pressure resistance or service life of individual components is in that case far below that which such a pipe or tube or component would reach with a static pressure loading.
Static pressure resistance of components depends inter alia on the mechanical properties of the material such as yield strength (Rp0.2) and tensile strength (Rm). Dynamic pressure resistance is determined by further parameters such as for example ductility (elongation to fracture A) of the material, the depth of incipient cracks in the tube wall, the degree of purity and the microstructure.
The failure of tubes and other components because of dynamic loading occurs due to critical crack growth, for example from the inside surface of the tube to the outside surface thereof. In that case cracks can occur due to local stress concentrations, for example at intermetallic inclusions, or incipient cracks which already exist are further propagated. The service life of a tube or another component under dynamic loading thus depends inter alia on the magnitude of the crack growth per pressure shock and the material thickness.
To increase the dynamic service life of components in a given loading collective, the above-mentioned influencing factors are optimised in production of the material and/or methods are used for subsequently improving certain material properties, such as for example autofrettage.
Autofrettage denotes a method of increasing the fatigue strength of components in high-pressure technology. In that case pipes or tubes and other components are loaded once and only for a few seconds to approximately 80% of their calculated bursting pressure, that is to say far above the calculated static operating pressure. Under that loading microplastic flow in the material occurs above the yield strength or elastic limit of the material and residual compressive stresses are produced in the substance of the material. When the component is relieved of load those residual stresses remain in the material and prevent or reduce the growth of incipient microcracks. As autofrettage only immaterially influences the mechanical properties of the autofretted material it thus has scarcely any influence on the static pressure resistance of the tube or component, but rather on dynamic pressure resistance.
A disadvantage of autofrettage is that this method is limited in respect of the required pressures and the static pressure load-bearing capability of the components to be autofretted. In some cases or for certain materials pressures of over 15,000 bars, for example pressures of between 18,000 and 22,000 bars, would have to be produced, by calculation, to achieve satisfactory results in the components, and that can only be implemented at the cost of extreme complication and expenditure in terms of installation technology, or at the present time cannot be implemented at all. Furthermore the level of the residual compressive stresses which can be locally produced is limited because the autofrettage pressure cannot be increased just as desired as otherwise the component would burst. The autofrettage method is thus technically limited, it cannot provide the pressures required on the basis of calculation at all for many materials and it is also highly cost-intensive and demanding.